Same-sex pairs of monogamous birds are just as attached and faithful to each other as those paired with a member of the opposite sex.

The insight comes from a study of zebra finches - highly vocal, colourful birds that sing to their mates, a performance thought to strengthen the pair's bond.

Scientists found that same-sex pairs of finches sang to and preened each other just like heterosexual pairs.

The study is reported in the journal Behavioural Ecology and Sociobiology. A displaying pair of king penguins Male king penguins have been seen to "flirt" with other males in the colony

Lead researcher Julie Elie from the University of California Berkeley said that the research showed that "relationships in animals can be more complicated than just a male and a female who meet and reproduce, even in birds".

Dr Elie and her colleagues are interested in zebra finches' behaviour. The birds establish life-long relationships and are highly social; males sing to their mates, the birds preen each other and pairs share a nest.